Samuel Gompers, the immigrant cigar maker who led the American Federation of Labor from the 1880s to the 1920s, argued that the purpose of political action by trade unionists was to "reward our friends and punish our enemies."

Today, when election endorsements by labor unions are often portrayed as little more than component parts of the broader bureaucracy of contemporary politics, the Gompers premise might sound old-fashioned. Yet it still comes into play now and again, as with the American Postal Workers Union endorsement of Bernie Sanders illustrates.

Sanders has the support of National Nurses United, the activist union that has been in the forefront of the fight for single-payer healthcare, along with endorsements from union locals in key states -- such as International Brotherhood of Electrical Workers (IBEW) Local Union 490 and Service Employees International Union (SEIU) Local 560 in New Hampshire. And now he has the 200,000-member postal worker's union.

The APWU endorsement recalls the historic "reward our friends" calculus, as Sanders has for years been an ardent congressional advocate for postal workers and the United States Postal Service. Long before he considered presidential politics, the senator from Vermont was arguing against the austerity economics that seeks to balance the books by cutting public services. An ardent foe of privatization, he as well has championed the expansion of its mission, backing innovative initiatives such as postal banking.

"We should judge candidates not by their political party, not by what they say, not by what we think they stand for, but by what they do," argues APWU President Mark Dimondstein. "Applying that criterion, Senator Bernie Sanders stands above all others as a true champion of postal workers and other workers throughout the country."

In announcing the endorsement, Dimondstein highlighted the candidate's broader pro-labor agenda -- "No other candidate has his record of standing with workers on picket lines, fighting for a $15 per hour minimum wage...and opposing 'fast track trade authority' and rotten trade deals like the Trans Pacific Partnership." But the APWU president and his union focused, in particular, on the senator's history as an advocate for the postal service and its employees.

Sanders, who has walked picket lines and rallied with postal workers for years, appeared before a cheering crowd of 2,000 APWU members in late October and celebrated the historic commitment of the USPS to serve every American -- "whether you are a low-income elderly woman living at the end of a dirt road in Nevada or Vermont or a wealthy CEO living on Park Avenue."

"The beauty of the Postal Service is that it provides universal service six days a week to every corner of America, no matter how small or how remote. It supports millions of jobs in virtually every sector of our economy. It provides decent-paying union jobs to some 500,000 Americans, and it is the largest employer of veterans," the senator declared. Yet, said Sanders, while it does this "at a cost far less than anywhere else in the industrialized world, [these] accomplishments have not shielded the agency from those who wish to dismantle it."

Sanders decried the fact that "the Postal Service is under constant and vicious attack," noting that "the same billionaires who want to privatize Social Security, Medicare and public education, also want to privatize the Postal Service."

Just as Sanders advocates for the expansion of Social Security, he argues -- with Massachusetts Senator Elizabeth Warren -- for the expansion of the postal service with initiatives such as postal banking. "At a time when more than 68 million lower-income Americans have no bank accounts or are forced to rely on rip-off check-cashing storefronts and payday lenders, allowing the Postal Service to offer these kinds of financial services would be of huge social benefit," says Sanders.

Dimondstein says APWU members are enthusiastic about the senator's emphasis not just on saving the USPS but on expanding its mission.

"Bernie Sanders is a fierce advocate of postal reform. He staunchly opposes postal privatization, and supports enhanced postal services, including postal banking," explains the APWU president. "Based on his Senate record, we are confident he will appoint good people to public office and end conflicts of interest. He has already blocked the slate of nominees to the postal Board of Governors that includes the 'king of postal privatizers' James Miller and payday lending industry lobbyist Mickey Barnett. No other candidate has his record of exposing the rule of the billionaire class."

The other contenders for the Democratic nomination, Clinton and former Maryland governor Martin O'Malley, have made strong pitches to labor. Both have joined union protests targeted at Republican front-runner Donald Trump. And Clinton has secured endorsements that are already helping her advance her candidacy in states such as Iowa. The fight for the support of unions and their members will go on. And the APWU endorsement offers a reminder that the race will be competitive and that, in some cases, decisions about endorsements will turn on questions of history and vision that have not always been recognized by political and media elites.

The fight to preserve and strengthen the postal service has gone on for many years -- often with scant attention from the media, and often with scant support from a Congress that has made it harder for the USPS to survive and compete. Along with Congressman Peter DeFazio, D-Oregon, Congressman Mark Pocan, D-Wisconsin, and a handful of others, Sanders has been a steady ally of the labor, civil-rights, rural-advocacy and small-business groups that have struggled to keep post offices open and to maintain strong, universal services.

John Nichols, a pioneering political blogger, has written the Online Beat since 1999. His posts have been circulated internationally, quoted in numerous books and mentioned in debates on the floor of Congress.